It is already known in the art to use optical resins as materials for optical conductors of distributed refractive index type, including optical fibers, optical waveguides, optical integrated circuits, and base materials (preforms) therefor.
The optical resins used for the optical conductors of distributed refractive index type are usually required to have a high level of transparency and an accurately controlled distribution of refractive index. For instance, in a distributed refractive index type optical fiber, it is necessary to form an accurately controlled radial distribution of refractive index in a resin material having a high level of transparency, in order to realize a broad transmission band and a low loss.
The techniques conventionally known for producing optical resin materials with distributed refractive index can be roughly classified into a copolymerization method and a diffusion method. None of the presently available techniques can produce optical resin materials that fully meet the above-mentioned requirements.
The copolymerization method is a method in which two or more different monomers are copolymerized. For example, Examined Japanese Patent Publication (KOKOKU) No. 54-30301 and Unexamined Japanese Patent Publication (KOKAI) No. 61-130904 disclose optical conductors having a refractive index gradient, which are produced by utilizing the copolymerization reaction of monomers having different reactivity ratios r1 and r2, and changing the proportion of the monomer units contained in the polymers generated in the early stage and the late stage of the polymerization.
This method makes use of the difference between the copolymerization reactivity ratios r1 and r2, and therefore, the greater the difference between the values of r1 and r2, the easier it becomes to produce a difference in refractive index; however, as the difference between the values increases, the monomers are more likely to be polymerized independently, compared with the rate of the copolymerization. As a result, phase separation unavoidably takes place between the reaction products. If fluctuation in the refractive index distribution is caused due to such phase separation, then a scattering loss occurs in the resulting optical conductor.
If an intensive phase separation occurs, it is observed as a cloudy phenomenon, which possibly makes it utterly impossible to use the material for applications including optical fibers, optical waveguides, and optical integrated circuits.
In addition, increasing the difference in the reactivity ratio between monomers causes a monomer having a lower reactivity to remain as monomer until the last stage of the polymerization process, and if the polymerization process is not complete, the resulting optical device is subject to change with time, which leads to deterioration of the characteristics.
In contrast with the copolymerization method, the diffusion method is a method wherein gel is produced from a monomer; another type of monomer is poured and diffused in the gel; and then the mixture is fully polymerized.
For example, according to a method proposed in Examined Japanese Patent Publication (KOKOKU) No. 52-5857, a polymer precursor (prepolymer gel), which has a transparent and three-dimensional crosslinked structure, is prepared first; then a monomer, which forms a polymer having a different refractive index, is diffused from outside; and the mixture is subjected to post-treatment to be fully polymerized and solidified, thereby producing an optical conductor having a distributed refractive index.
According to this method, however, it is necessary to prepare the polymer precursor in advance. In addition, the polymer precursor requires that the polymerization be interrupted in a middle stage of polymerization, but it is not easy to stop the polymerization reaction with good reproducibility at a stage where a certain conversion ratio is reached. Dispersion in the conversion ratio affect the subsequent monomer diffusion process, making it difficult to accurately control the distribution of refractive index of the optical conductor.
Furthermore, the polymer precursor, in order to maintain its shape, needs to be produced using a polyfunctional monomer which has crosslinking properties, or be produced as a copolymer composed of a monofunctional monomer and a polyfunctional monomer. However, a polymer containing a crosslinkable monomer tends to exhibit poor thermoplasticity, making the treatment after the polymerization difficult. This drawback is not negligible. Especially when producing an elongated optical conductor such as an optical fiber, this method is extremely disadvantageous in that a process involving hot drawing is hardly applicable.